This invention pertains to methods for removal of pollution from the exhaust gas stream of combustion sources. More particularly, the invention relates to an improved method of elimination of particulate matter, sulfur dioxide (SO.sub.2) and volatile toxic materials from the flue gas of coal fired boilers, especially in large electric generating plants.
Because of increasing public concern over deterioration of the environment, industries are being required by current and anticipated regulations to limit the emission of particulate matter, toxic materials (e.g., volatile metals or organic materials), and precursors to acid rain (SOx, NOx) into the atmosphere. Unfortunately, many processes for controlling these pollutants are either unproven or expensive. Known SO.sub.2 removal and collecting processes include wet scrubbers, spray dryers, and dry furnace sorbent injection. Modern methods of collecting particulates include fabric filters (baghouses), electrostatic precipitators, and wet scrubbers. Methods of collecting volatile toxic materials are not well established, but are generally based on scrubber or filter technology. A discussion of each of these systems follows.
In wet flue gas desulfurization systems, also called wet scrubbers, small droplets of slurry containing water and alkaline material are sprayed into the flue gas, where it reacts with SO.sub.2 and the reacted products are collected together as a liquid. Wet scrubbers have high collection efficiency and are the only systems considered commercially available for over 90% SO.sub.2 removal on boilers firing high sulfur fuels. This system has the disadvantages of large capital costs, high maintenance costs, and the need to dispose of the solid wastes produced by the process. Also, wet scrubbers may emit a significant plume of evaporation residue and H.sub.2 SO.sub.4 into the atmosphere.
Spray dryers are another system of sulfur removal, wherein fine droplets of slurry are sprayed into the flue gas to react with SO.sub.2 and then evaporate to become airborne solid particulate residue. This residue is then collected in a baghouse in which the flue gas is filtered for removal of particulates. Spray dryers are lower cost then wet scrubbers, and generate less sludge, but their performances may not be adequate for high-sulfur fuels. Furthermore, disposal is complicated by the addition of sorbent to the collected ash particulates. Also, the flue gas temperature is lowered dramatically by the evaporation of water which may lead to corrosion of baghouse components. Water consumption by spray dryer processes are nearly equivalent to those for wet scrubbers.
Dry sorbent injection is a third method of sulfur removal. An alkaline powder is blown into the flue gas and reacts with the SO.sub.2. The reactants are then collected in an electrostatic precipitator or a baghouse, where the sorbent may react further with the SO.sub.2. The capital cost of such a system is significantly lower than that for scrubbers or spray dryers. However, the SO.sub.2 removal efficiency is lower than for wet scrubbers and spray dryers.
The predominant system of particulate removal at coal fired power plants is the electrostatic precipitator (ESP). An ESP consists of a series of pairs of electrodes maintained at a high voltage difference within a gas passage for ionizing the air. As dust or ash laden air passes between the electrodes, the particles are also ionized and move in the electrical fields to be collected on one of the electrodes. The material that is collected is periodically dumped into hoppers by mechanical means and removed to a disposal site. ESPs have been selected as the particulate removal means in the past due to their low cost. However, with more stringent removal requirements, alternate technologies have become competitive, since ESPs are extremely sensitive to dust properties (i.e., coal types).
Baghouses consist of an array of tubular fabric filterbags within a large housing through which flue gas containing dust or ash is drawn by a large fan. The filterbags are cleaned periodically be reversing the gas flow and/or shaking. The particles are collected in hoppers for disposal. Baghouses are relatively insensitive to the properties of the particulates and have a very high collection efficiency.
Wet scrubbers may also be utilized to remove particulates, but more energy is required than for sulfur removal. At present, scrubbers are not efficient enough to compete with ESPs and baghouses.
Many toxic materials, such as lead, selenium, arsenic, etc. have the property that they are in vapor phase at flue gas temperatures, but condense at atmospheric temperatures to become solid or liquid particles, a form potentially hazardous to human health. As vapors, these materials may pass through baghouses and precipitators virtually unaffected. In scrubbers, they condense because of the lower temperatures and then pass through as solid or liquid particles because of the low collection efficiency. The common means of collecting such materials is to reduce the flue gas temperature by some means, condense the material to particulate form, and then collect it in ESPs or baghouses. This can be done by using "wet" ESPs wherein water is sprayed into the inter-electrode space to cool the gas, or in regular ESPs and baghouses by preceding them with spray towers or heat exchangers to lower the temperature of the flue gas and cause the vapors to condense. These techniques waste heat energy if water is used and may cause low-temperature corrosion in downstream components if either water or heat exchangers are used.